The present invention relates to a charging apparatus and a charging system and, more particularly, to a charging apparatus and a charging system for charging a secondary cell using an unstable electric power supply, such as a solar cell, which generates variable voltage, and for properly detecting the fully-charged state of the secondary cell.
A charging apparatus for charging a secondary cell, such as a lithium ion battery, using an unstable electric power supply, such as a solar cell, compares an output voltage from the solar cell to a reference voltage, then starts or stops charging on the basis of the comparison result, since the output voltage from the solar cell changes. Further, the charging apparatus also detects a fully-charged state of a secondary cell based on a decrease in an amount of charging current and a timer, similar to a charging apparatus for charging a secondary cell at a constant voltage in current-limit method using AC power supply.
In the aforesaid charging apparatus, however, there are the following problems.
First, in the aforesaid charging apparatus, the range of voltages within which a secondary cell can be charged is limited. Therefore, it is not possible to charge the secondary cell when the output voltage from the solar cell is less than a reference voltage due to weather conditions, such as when clouds block insolation, for example. Second, when the output voltage from the solar cell changes rapidly, starting and/or stopping charging operation causes further change in the output voltage from the solar cell. Under this situation, a function for supervising an input voltage (output voltage from the solar cell) to the charging apparatus may not operate correctly.
Further, in the aforesaid charging apparatus, when charging current decreases in response to a drop of output voltage from the solar cell, fully-charged state of the secondary cell may be erroneously detected. Furthermore, when the charging operation is finished in accordance with the timer after charging the secondary cell by a smaller charging current than expected, the secondary cell may not be fully charged.